The present invention relates to stepping-type magnetic mechanisms and, in particular, to control-rod drive mechanisms of the magnetic-jack variety.
As is well known in the art, nuclear reactors are controlled by inserting and retracting neutron-absorbing control rods into and out of the fuel-bearing core of the reactor. Generally speaking, the rate of reaction is increased by retracting control rods and decreased by inserting them. The most popular method of moving control rods is to use magnetically actuated control-rod drives. The drives move an extension of the control rod by alternately moving one or the other of two latches. In a typical case, each latch would be operated magnetically by two coils. One of the coils would operate to cause the latch to engage the control-rod extension, and the other would operate to raise the latch once it is engaged. The rising latch would raise the control rod with it. Another latch would then engage, the first latch would disengage, and the rod would then be raised by the second latch. Thus, through a sequence of coil actuations, the latches could be made to work in a hand-over-hand manner, thereby retracting or inserting a control rod.
Though the popularity of this method attests to its effectiveness, room for improvement remains. One of the difficulties encountered in the operation of the magnetic jack is that it requires a trade-off between effective operation and longevity. Effective and reliable engagement of the latch can best be afforded when the force caused by the engagement coils is relatively high; whatever tendency there may be for sticking is then more likely to be overcome. On the other hand, longevity is best afforded when the impact of the latches on the control rod is low, and this calls for a lower coil force. Avoidance of sticking is also avoided by relatively high coil force in the lifting step, but the impact between magnetic-jack parts is greater when the lifting-coil force is relatively high, so there is again a trade-off between effective operation and longevity.
It has been suggested that the wear problem could be reduced somewhat by using a two-level drive voltage. The higher voltage would be used at the beginning of the actuation, thereby causing the greatest force when the sticking might occur, and the lower drive voltage would be applied after the higher force had been applied for a fixed time, thereby somewhat reducing the force that impels the latch into contact with the control-rod extension. While this approach is not totally ineffective, there is still usually some overlap between high coil force and latch movement.